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Re: [Phys-l] pseudo-force


Force perception is not always the same as force.

I did some surveying on this topic with highschool & elementary students, and some physics teachers/professors as part of a methods class project a few years back.


The results surprised me:

1) Forces that result from active physical contact from the person will _ususally_ be perceived in the proper physics direction.
Forces that result from passive physical contact are _often_ perceived in reverse from their correct direction.

2) The more you know about forces the more you are likely to change the way you perceive the forces.

3) Everyone lives in their own private universe and perceives situations differently.


Examples:
A person riding in an accelerating car. They feel pushed back since riding is mostly a passive activity. This is the way most beginners see the problem.

If the problem is recast as person holding on to the bars of a motorcycle or the horn of a saddle on an accelerating horse, the force is perceived in the correct direction.

A person riding in a car rounding in a corner perceives an outward force. The same person holding on to a handle on a playground ride that spins in a circle will report they are pulling inward.


I used a battery cartoon survey situations similar to these and got similar but not consistent results. It may turn out the 'we' have changed our perceptions of what we feel to allow for what we know about force interactions. A physics trained person is more likely to feel the car seat pushing them forward rather than back. Might explain how we can draw free-body diagrams correctly and our students struggle.

I thought about extending the survey and putting a paper together, but was discouraged since no clear cut results were found.

In the end pseudo-forces end up being used to describe situations where inertia is being cast a tangible thing capable of creating forces. This creates an interesting mix of force perceptions and forces. IMHO, this is a very weak way ( but sometimes convenient) to view a problem and will cause endless confusion later as students must now decide if inertia will create a force in a given situation or not. I avoid it completely. Forces result from interactions with other objects. Inertia cannot create forces, it decides what acceleration forces create. Makes FBD's so much easier.


Scott




*******************************************
Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
sgoelzer@coebrownacademy.com
*******************************************


On Oct 27, 2006, at 3:58 PM, Bernard Cleyet wrote:

"When you put a hat on, you notice the forces, but after a while you can hardly notice
the hat."

And the converse.

bc, and sometimes thinks he's still wearing the hat or ear aids.



Folkerts, Timothy J wrote:

I would tend to say that what you "feel" is not "force" per se but the
distortions they cause to your body.

* Gravity pulls on all parts of your body and hence doesn't cause any
distortion and hence you don't "feel' it. But if you are standing on
the floor, the soles of your feet compress, causing the sensation of
force.

* Air presses uniformly on your skin from all sides, which creates a
"typical" compression that your body ignores. If the pressure to a
particular spot either increases or decreases, that creates is a
distortion that registers as a force on that section of your body.

* If you hold an object in your hand, the area under that object gets
compressed and you "feel" the weight of the object.

* if a force remains constant long enough, the body becomes accustomed
to the new equilibrium, and starts to ignore the force. When you put a
hat on, you notice the forces, but after a while you can hardly notice
the hat. (that might partly explain why low density objects tend to
"feel" lighter than high density objects of the same mass - at least
they do to me. When the force is quite spread out, there is little
disortion to the skin and you quickly ignore the distortion. A more
concentrated distortion is harder to ignore.)


Tim F
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Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l